The F-Plasmid

F-Plasmid:

The F-plasmid of E. coli is a self-transmissible, low copy-number extra-chromosomal genetic element mediating its own transfer. The transfer process requires products of a good number of genes (about 40) which are located in the plasmid itself. Among the more important functions of these genes are those relating to the replication of the plasmid, production of sex or F-pili and the formation of a mating bridge between the conjugating F+ and F– cells.

The F-plasmid is a comparatively large double- stranded circular DNA molecule having about 100,000 base pairs and a molecular weight of about 63 x 106 Daltons. The plasmid replicates once in each cell cycle and the daughter molecules are segregated to daughter cells. Number of copies per cell is one or two.

Transfer of F-plasmid from a donor to a recipient is accompanied by replication of the plasmid. When the two conjugating cells come in contact and a mating-bridge has been formed, the plasmid replicates by the rolling circle model and a single-stranded copy is transferred to the recipient.

New DNA is synthesized in both the donor and the recipient, so that the plasmids in both cells become double stranded and circular by sealing the open ends through ligase. When a small number F+ cells is mixed with a large number of F– cells, all cells become F+ after some time, because the F– cells become F+ and can act as donors.